1. Allow cream cheese to come to room temperature, so it will be easier to blend.
2. Preheat oven to 350° Fahrenheit (177° Celsius). Fill any baking pan about two thirds of the way up with water, then set this pan aside.
3. Prepare crust of choice (if using a crust) in an 8 or 9 inch springform pan.
4. In a large food processor, or in two batches in a smaller food processor, beat softened cream cheese, yogurt, protein powder, sweetener, lemon juice and zest, and vanilla extract just until smooth. Do not over-whip, which could introduce air bubbles and cause cracking.
5. Spread the lemon protein cheesecake filling into your prepared crust or the bottom of a greased springform pan for a crustless option. Smooth out the top.
6. Place the baking pan of water on the lower rack of the oven, then place the cheesecake pan on the center rack. Bake 30 minutes.
7. Without opening the oven, turn off the heat and let the recipe sit in the closed oven another 5 minutes. This allows the cheesecake to cool down slowly.
8. Remove from the oven and cool fully on the counter. Then refrigerate uncovered for at least 6 hours or overnight, during which time it will firm up considerably.
9. Slice and enjoy. Refrigerate leftovers in a covered container for up to 5 days, or freeze for longer term storage.